The invention relates to a method of smoothing out the thickness of a sheet of thermoplastic material which is extruded through a sheet die, in which method the current thickness of the sheet is measured over the sheet width and the measured thickness values are averaged within equally wide sheet sections, and also a device for this purpose.
In the production of two-dimensional plastic structures, such as, for example, sheets or panels, the plastic melt is generally remolded from a circular or oval cross section into an approximately rectangular cross section by a sheet die. In this process, the sheet die delivers a sheet preform profile having a cross section which is not precisely rectangular. The profile of this sheet preform, which is frequently convex, is adjusted by suitable controlling elements at one of the die lips of the sheet die with whose aid the die gap is controlled. The sheet preform delivered by the sheet die is then biaxially-stretched in order to obtain a final sheet having a flat transverse thickness profile. This final sheet is, consequently, a biaxially oriented sheet of plastic, for example polypropylene or polyester, which is molded by longitudinal stretching by means of rolls and transverse stretching in a transverse stretching frame.
During the operation of a sheet or panel extruder, the final sheet profile is subject to certain variations due, on the one hand, to raw material inhomogeneities in the plastic melts and, on the other hand, to deviations in the process parameters from specified set point values. For a flat transverse thickness profile of the final sheet to be achieved despite this, interventions are necessary at the die lip of the sheet die. These interventions are undertaken during the continuous operation of the machine and are essentially carried out in one of two ways, namely:
(1) Modifying the thickness profile by differently heating the die body of the sheet die, mainly in the vicinity of the die lip, as a result of which the melt viscosity is altered, under which circumstances a locally reduced viscosity results in a reduced frictional resistance and, consequently, in a higher material throughput of the plastic melt through the gap of the sheet die if the pressure drop is the same, and PA1 (2) Modifying the thickness profile by adjusting die bolts which generally interact with the upper die lip in order to alter the die gap width. The die bolts may, for example, be adjusted manually in order to exert pressure or tension on specified regions of one of the die lips. Instead of mechanically adjustable bolts, use is very often made of so-called thermobolts which adjust the die gap by thermally adjusting specified regions of one of the die lips. Electrical power is supplied to the thermobolts so that they expand more markedly or less markedly and thereby exert pressure or tension on the specified regions of the die lip.
As explained above, this adjustment is carried out either mechanically and manually by means of positioning motors or by means of thermobolts. In practice, it is found, however, that however precise the adjustment of the thickness profile, thick and thin points which can no longer be modified remain behind in the final sheet. This may make further processing of the final sheets difficult or even impossible. The ring-shaped beads which form in the sheet rolls impair the flatness of the unwound sheet web, with the result that faults may occur in further processing of the sheet web.
German Patent No. 3,006,566 discloses a method of smoothing out the sheet thickness of flat sheets produced in flat-sheet extrusion lines, in which method the current thickness of the film is measured in equally wide sheet sections, and correction sections of the sheet die from which these have been extruded are allocated to the sheet sections. In this method, the correction sections are exposed to different temperatures/heating powers in accordance with the measured thickness deviations of sheet sections from a thickness reference value, and specifically to an extent determined by the deviation from the thickness reference value in each case. Either the greatest thickness or the smallest thickness of one of the sheet sections is chosen as thickness reference value. If the sheet section exhibits a greater thickness exceeding the thickness reference value, cooling is carried out, i.e., the heating power is reduced whereas in the case of smaller thickness, heating is carried out, i.e., the heating power is increased.
European patent application EP-A 0,330,603 describes a device for extruding thermoplasticized plastics through a sheet die having a die gap which is determined by two flank faces of a die body, one of which is designed as a flexible die lip onto which a plurality of lever arms are molded. Each lever arm can be conveyed to a limited extent around an axis of rotation extending parallel to the die gap by an adjustment screw passing through the lever arm or by a similar mechanically constructed adjustment device in order to alter the gap width of the die gap.
In the method of producing a sheet of thermoplastic material with the aid of a sheet die described in German Patent No. 3,740,087, the controlling elements provided are so-called piezo translators which are controlled by control variables of a control device. In the event of a deviation of the actual value of the sheet thickness in a width section of the sheet die from a specified set point value, a suitable controlled variable is applied to the piezo translator allocated to the width section.
According to German Offenlegungsschrift DE-A 3,834,719, the die gap of a sheet die is adjusted by means of air-cooled and electrically heated thermobolts which carry a claw at one end and an adjustment screw at the other end. The claws embrace a bar-like die projection near the upper die lip of the sheet die and are linked to the die projection by means of detachable tapered pins. The adjustment screws of the thermobolts are screwed into a threaded block which is in turn screwed to a shoulder of the die body.
Common to the known methods and devices for extruding thermoplastic sheets is the fact that, however precise the adjustment of the thickness profile of the sheet preform, thick and thin points remain behind which make themselves felt even in the biaxially-stretched final sheet. These thick and thin points can no longer be smoothed out, even by biaxial-stretching.